1. Field of the Invention
The present invention relates generally to interferometric spectrophotometers such as Fourier transform infrared (FTIR) spectrophotometers, and more particularly to a method and apparatus for controlling the reciprocation of a movable mirror of a double-beam interferometer used in such interferometric spectrophotometers.
2. Description of the Prior Art
Known double-beam interferometers include, additional to a main interferometer for producing an interferogram, a control interferometer for purposes of monitoring the reciprocating movement of a movable mirror and sampling the interferogram.
Detectors for use in Fourier transform infrared (FTIR) spectrophotometers include a pyroelectric detector such as TGS and LiTaO.sub.3, a quantum detector such as MCT and InSb, and a photoacoustic (PAS) detector. In the interferometers, the speed of reciprocation of the movable mirror represents a modulation frequency. The optimum modulation frequency, however, is not the same for all such detectors but varies depending on the type of the detectors. Furthermore, in the PAS detector, different speeds of reciprocation of the movable mirror result in the detection of pieces of information having different depths from the surface of a sample.
In some FTIR spectrophotometers, the reciprocating speed of the movable mirror is variable, however, number of variable speed ranges available in such spectrophotometers are several at the greatest.
Due to the impossibility of changing the speed of mirror motion through multiple stages, the conventional FTIR spectrophotometers are unable to attain an effective use of a photoacoustic Fourier transform spectroscopy using the PAS detector which is capable of detecting different pieces of information in reliance upon different speeds of mirror motion.
In the mass-production of the interferometric spectrophotometers, the reciprocating speed of the movable mirror cannot be set such that each individual interferometric photometer has a particular maximum signal-to-noise (S/N) ratio.